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Old 30th April 2013, 03:23 AM   #1
hifiZen is offline hifiZen  Canada
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Default UL / Distributed Load - Screen vs. CFB Winding Ratios

... so I have a bunch of these nice "super" 6BG6-GA (7027A in disguise) gathering dust, just burning for me to build a nice amplifier around them.

For this design, I want to try zero global feedback, which means extracting the maximum performance from the output configuration & local feedback. Distortion trumps power, but I'd still like to get reasonable efficiency. Triode class AB2 is one option, but I'm thinking I would rather run ultra-linear, with a separate screen winding to allow a higher B+ on the plates, and another separate winding for cathode feedback. Having not tried the latter before, and wanting to 'get it right' (or close to right) with a set of custom-wound OPTs, all sorts of questions about distributed load winding ratios come to mind…

For a start, the optimal Pentode loading appears to be 6.6k, which SY also suggests is a good loading for Triode strapped operation. One might then assume then that this would also be a good load for UL operation - would that be a correct statement?

Second, I'm having a hard time figuring out the optimal UL% to use. I am presently reading R. Moers' article on calculating the optimal UL%, but it seems that 43% is commonly recommended by the original manufacturers for this broad family of valves, apparently on the vague premise that about 25% gives most of the distortion reduction, but there is more to gain with yet more screen feedback. I am also uncertain how the screen FB would be affected, if at all, by running a separate winding at a lower DC potential than the plates.

Third, I haven't yet decided how much winding to put under the cathodes. What Crowhurst calls "modified ultra-linear" subtracts the full UL percentage from the primary and places it in the cathode winding, such that the screens simply tie straight to a regulated supply. This achieves the maximum cathode feedback without reversing the polarity of the screen drive (and neatly eliminates one winding), but at a typical UL ratio of 43%, it demands an awful lot of voltage from the driver stage. Drive swing and heater breakdown considerations may set a practical upper bound, but I wonder if there is an optimization / distortion null (maybe an IMD minima?) to be obtained by choosing the right ratio of cathodes to screen winding, or if the benefit of the larger 43% vs. a lesser UL% is largely wasted, unnecessarily burdening the drive stage? I note that the Quad II apparently uses only a 10% winding at the cathode.

Finally, in some distributed load implementations (MacIntosh, etc.), I have seen the appearance of an extra small-value choke at the plates of the output tubes, and I'm curious as to the purpose of this. Stability? Reduced IMD?

Between plate load impedance, bias point, plate & screen supply voltages, screen winding % and cathode winding %, I don't think I could possibly explore the universe of possible solutions on my own. I'll keep reading as much reference material as I can lay my hands on, but I'm hoping the resident experts will come out of the woodwork and help guide me towards a good configuration for some custom OPTs.
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Old 30th April 2013, 05:03 AM   #2
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43% of voltage (hence also number of turns) has been the accepted "standard" for Ultralinear taps. If you go back and start researching the beginnings of Ultralinear you will find that 43% is the optimum minum distortion configuration for EL34 tubes (20% for max power). Also appropriate for EL84. Once you start talking about other tubes then other ratios actually apply but since these are hardly ever documanted folks just assume that 43% will suit every tube.
An old paper by Fritz Langford Smith (Editor of Radiotron Designers Handbook) suggests 5 to 10% is what best suits 6V6. I have seen seen at least one KT88 amp which used 50% taps.

Next you state you want to combine cathode feedback and ultralinear. The thing to recognize here is that the Ultralinear feedback is the voltage applied between screen and cathode. If you apply 10% NEGATIVE cathode feedback then the ultralinear taps will want to be around 30% to give you the same Ultralinear operation as 10 + 30 = 40%.

If you look at Plitron VDV2100 CFB/H Torroidal Output Trannies (for example) you will see that it offeres exactly this, 10% cathode feedback windings with UL taps at 30%.

They also have a 2100-SSCR which is 40% Ultralinear in a separate winding.

There are a few weird arrangements around, probably the weirdest is the Jadis JA80 (4 x KT88 in Class A for 80 Watts) which uses about 50% Ultralinear with 10% POSITIVE cathode feedback to arive back at the approximately 40% point again.

If you want the Fritz Langford Smith articles (From Radiotronics Magazine) send me a PM. They would be too big to post here (approx 500KB). They were in 3 parts, a quick google of "Radiotronics Ultralinear" suggested that a couple of the parts may be available on some sites.


Cheers,
Ian

Last edited by gingertube; 30th April 2013 at 05:07 AM. Reason: Deleted a dead link
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Old 30th April 2013, 08:47 AM   #3
hifiZen is offline hifiZen  Canada
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Thanks Ian,

I'll send you a PM about the paper - I may already have it, but if not I would definitely like a copy. Over the years I've been collecting papers on Ultra-Linear / Distributed Loading from around the web. The two best sources I've found recently are the Pearl Audio archive and this page.

I still need to digest some of these articles, but I think I have the basic winding rules figured out... To summarize: cathode winding + plate winding are in series driving the load, so whatever percentage winding is used at the cathode must be subtracted from the plate winding in order that the tube sees the same total load impedance. The cathode winding will also subtract voltage from Vg2-k, so the cathode winding percentage should also be subtracted from the UL tap or winding, as you note. However the screen grid does not really participate in driving the load, and so the percentage selected for the screen connection does not really affect the other windings.

Where I could use some help is in the optimization...

Giving my last post a bit more thought, let me try and be a bit more methodical than just posting a laundry list of questions... I'll start with the most essential one: does anyone have any data or experience on what the optimum UL ratio is for the 7027A / super-6BG6 is?

I may need to go make some measurements to figure that out, though I don't have all the bits 'n bobs needed to do that just yet. It seems to me that once the basic tube characteristic is linearized with g2 feedback, then any other feedback applied (including cathode feedback) will simply reduce distortion further. So I suspect if I can get the UL ratio tuned well first, then deciding what cathode feedback ratio to use will be a much simpler matter, provided there isn't some hidden gotcha or overlooked advantage lurking.
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Old 30th April 2013, 09:35 AM   #4
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As suggested here:
Best PP OPT 30-100W range
the OPT you want is not going to be cheap. Tertiary windings require quite some additional work and re-design but I think they are worth the price and represent the best solution even if you had tubes which usually do not require lower G2 voltage.
For cathode feedback you can ask for 10%. This should be enough to drastically reduce plate resistance even in pentode/tetrode connection. The amount of dB's depends on the tube and operations. However it should be in the range between 6 and 12 dB.
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Old 2nd May 2013, 09:17 AM   #5
hifiZen is offline hifiZen  Canada
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Thanks 45.
I know the OPTs won't be cheap, but I consider it a good place to put the budget saved by going with an oddball NOS tube instead of the high-demand / high-dollar audiophile stuff (300B, KT88, etc.). I think the investment is worthwhile - a good OPT and output tube are the foundation of the amplifier ... the preceding stages and power supply circuits can be changed around at relatively little expense.

10% sounds like a sensible value for the CFB windings.

I'm now reading the "Adjustable distributed load discussion" thread, so maybe I'll decide to try and measure my tubes with a transformer on hand, in the search for that optimal UL ratio.

-----

Anyone have any insights into the little plate chokes?

See attached schematic of the McIntosh MC40 output stage, which shows 2.2uF chokes. Strictly for stability, or is there more to it?
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File Type: png McIntosh MC40 Output Stage.png (107.2 KB, 517 views)
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Old 2nd May 2013, 11:22 AM   #6
JLH is offline JLH  United States
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CFB works best with high prevalence, high Mu tubes. It is a complete waste to use CFB with low to medium Mu triodes. The point of diminishing returns (required driver vs. lowered Rp) falls between 10% to 15% for most pentodes. You can go upwards to 20% with low prevalence, low amplification pentodes. However, your drive requirements can get ridiculous pretty quick if you don't think things through. I love CFB with high prevalence sweep pentodes. For example, a 6JG6 with 15% CFB basically becomes a triode with an Rp of 640 ohms and a Mu of 6.3
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Old 2nd May 2013, 03:38 PM   #7
TheGimp is offline TheGimp  United States
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When one says 10% CFB, is this 10% of the anode voltage and thus 10% of the primary turns?

The Langford-Smith articles are on the Pearl web site.

Last edited by TheGimp; 2nd May 2013 at 03:49 PM.
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Old 2nd May 2013, 05:39 PM   #8
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Chad: first the chokes, I'm pretty sure that their reason for existence, being so small in value, is to control spurious oscillations and/or HF ringing, and not fundamentally part of the architecture of the Mac.
I've been conducting lots of simulations and analysis of different kinds of beam pentodes and have come to the conclusion that there is a first order relationship between the optimum plate/G2 "swing" ratio and the ratio between of the gm between G1 and G2. You have already found in the literature from the Golden days that the 43% (impedance, mind you, not voltage ratio! i would prefer these stated in terms of voltage ratio as being far more useful in analysis) number is far from universal. Sweep tubes, for example, have a very high gm for G2 and intuitively I would assume them to want a lower ratio of G2 swing vs plate swing because of the far greater effect on plate current that G2 swing has.
Good luck, I'll be watching posts here, very interesting!
Rene
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Old 2nd May 2013, 06:18 PM   #9
JLH is offline JLH  United States
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Quote:
Originally Posted by TheGimp View Post
When one says 10% CFB, is this 10% of the anode voltage and thus 10% of the primary turns?

The Langford-Smith articles are on the Pearl web site.
It is a percentage of the primary turns, thus a voltage ratio. If we are talking about a SE transformer, then there is an extra tertiary winding that has X% of the whole primary turns. With PP transformers, there is normally a center tapped tertiary winding that is X% of the whole primary winding.
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Last edited by JLH; 2nd May 2013 at 06:19 PM. Reason: spelling error
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Old 2nd May 2013, 07:53 PM   #10
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It easy in our minds to expect too much of an "optimum percentage" for the screen taps. Each of the functions, distortion, output impedance, and power output, changes monotonically. There really isn't some "optimum" general choice.

As far as the McIntosh amplifier, those small chokes are for parasitic suppression. The output valves are biased almost off, so will try to excite primary resonances regularly during the duty cycle. The other polarity valve's cathode winding is bifilarly coupled, so helps to damp this, but no coupling is perfect, especially anode to anode.

All good fortune,
Chris
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